The present invention relates to an improved apparatus for supporting and driving a print head carriage across a printing media so that dots may be placed thereon to form a visual image to a human viewer. Prior art mechanisms for driving a print carriage typically utilize a belt and pulley actuated mechanism or helical gear attached to the print carriage. As digital print head accuracy and acceptable manufacturing tolerance specifications have improved, a limitation in field-replaceable precision mechanical driving mechanisms for such print heads has arisen.
Prior art print head rail members for large format print engines further encounter limitations due simply to the length and mass of a typical rail and carriage drive assembly and control difficulties related to precisely controlling all the specifications and tolerances during manufacturing and installation. A known phenomena described as "tolerance stacking" contributes a significant component of error in an assembly process wherein at least two precision machining events occur at differing times on the same assembly. In relation to a carriage assembly for a large format print engine, such tolerance stacking occurs at a number of discrete points of manufacture. For example, a carriage typically precisely supports at least one, and oftentimes several, print heads, a portion of the circuitry for such print heads, and attachment means for driving the carriage assembly upon a guideway or track in a highly controlled manner. Thus, machining portions of subassemblies to arrive at an exact location of the print head(s) relative to: other print head(s), the printing medium, and the rail assembly can all contribute positional error relative to a design criteria possessing rigorous tolerance specifications.
Additional problems with prior ink jet head configurations involve the mounting of the print head for accurate placement and movement across the printed image. The rail structure for the print head must adequately support the print head not only over the entire printed image, but also for any cleaning, maintenance and other auxiliary functions of the print head. It is common to provide a zone, away from the printing medium within which to "park" the print head to perform auxiliary "service" functions, this zone is commonly known as a service station. These auxiliary functions may include manipulating the carriage, certain calibration functions, cleaning and capping of the print heads. To accommodate the park zone, the support system, or rails, must support the head over a distance greater than the width of the printing medium. For example, printers handing printing medium about 11 inches wide (which accommodates the length of standard 81/2.times.11 paper) may have rails about 17 inches long.
Accurate placement and movement of the print head becomes more and more difficult as the length of the print scan (i.e., the width of the image) increases. Most prior ink jet printers over about 17 inches wide employ either a two-rail structure, or a single-rail and outrigger structure, for head carriage X-directional travel. Both of these techniques provide two separate and independently adjustable support points for the carriage. Multiple support systems were used on wide format printers because it was believed that a single rail could not provide adequate support and stability for the print head over a large distance. Multiple support systems were utilized to provide a wider support base for the print head and carriage to lessen the effect of any stability problems, as well as to provide additional strength to lessen rail flexing problems. Vibration problems may occur if the print head undergoes movement with respect to the rail structure. The print head may slightly rotate or shake about an axis parallel to the rails, causing the print head placement with regard to the paper surface to be inaccurate. Alternatively, the print head may slightly rotate or shake from side to side on the rails, perhaps due to the direction of print head travel.
Dual support systems are not altogether feasible for graphics quality, large format printing because it is difficult to maintain parallelism of the supports across the entire width of the large format media. More particularly, each support introduces positional error, resulting in non-parallel guide paths for the carriage. Further, prior art two-rail systems employ a pair of circular rails, with the print head mounted on a carriage which is in turn mounted on the rails. The carriage is generally supported by circular sets of ball bearings wrapped around each of the circular rails. Non-parallelism of the rails introduces vibration through the ball bearings to the carriage, often causing instantaneous horizontal velocity errors. If the supports are not parallel, the rollers on the carriage will bind or have excess freedom at particular locations along the rails, and cause further stability and vibration problems. If bending of the rails occurs and the railings are not maintained completely straight, errors occur in positioning the print head. Additional problems occur due to the space that the rails take up, interfering with the transfer of electronics and ink from the printer housing to the print head. It will be appreciated that these problems are magnified as the length of the rail or rails becomes greater, as in large-format printing. Accordingly, a print head configuration is desired which will avoid these various problems.
One mechanism for cleaning the print head involves wiping the print head with blotter paper as described in U.S. Pat. No. 4,928,120 to Spehrley, Jr., et al. The Spehrley, Jr. blotter is provided in a replaceable plastic module. The Spehrley Jr. blotter has a top roller for pressing against the print jet orifices and a bottom roller for pressing against the bottom face of the print head when they are being wiped. While this blotter works acceptably, a less expensive method and apparatus for blotting is desired.
Furthermore, such prior art carriage and drive systems typically are not designed for in-field replacement with minimum personnel and requiring a minimum amount of service time. In fact, due to the obvious competing design objectives of mechanical positional accuracy and field replacement convenience, the inventors are aware of only one other such rail system offering similar design benefits. The inventors refer to U.S. Pat. No. 5,592,202, and titled "Ink Jet Print Head Rail Assembly" which Patent is commonly assigned with the present invention to LaserMaster Corporation of Eden Prairie, Minn. 55344. In the application cited, a single rail pivotably attaches to at least one end of a print engine chassis so that the carriage riding thereon may be removed for field service and replacement. The benefits of such a system relate to diminished down-time, reduced required service, and efficient repairs, thereby reducing the overall cost of ownership involved in operating one or more large format digital print engines.
Prior art digital printing systems typically operate by incrementally moving or "stepping" a print medium transverse to a stationary or reciprocating imaging print head. The print head frequently includes a plurality of discrete imaging elements suitably arranged in a pattern, one or more linear arrays disposed perpendicular to the direction of movement of the printing substrate, or as a single marking point element. The net result is that discrete dots are placed on precise locations on the printing media so that a pleasing visual image is rendered upon the printing media. A picture element or pixel generally refers to a coverage area defined by this stepping resolution in the "vertical" or y-direction relative to a print head fixed in the x-direction, and the number of discrete marks producible by the thermal print head in the "horizontal" or x-direction. These pixels must be controlled very carefully to impart desired quality of the image, and the physical and chemical interaction between the marking material and the printing media and the environmental conditions under which the marking material is deposited upon the printing media all contribute to the quality of the actual image rendered.
Most digital print engines that typically use one or more of a subset of the four subtractive primary colors: cyan, yellow, magenta and black ("CYMK") and rely upon color blending of these four ink colors to achieve accurate representations of desired color(s). Upon combining ink colors at a given pixel that a particular color combination can be formed by having multiple ink colors at a particular pixel location, either in a dot-on-dot or a dot-next-to-dot configuration. In sum, digital printing processes involve placing a number of tiny dots onto particular locations on a printing medium. Any number of these small dots, when viewed some distance away from a printing medium such as film or paper, are perceived as a continuous-tone visual image. Thus, it can be appreciated that even slight variance in the actual positional location of the ink dots can significantly effect the overall visual impression created by the printed image. In one subset of digital printing technology, aqueous ink is expelled from a plurality of ink jet nozzles to form dots on the printing media. This is known as "ink jet" printing and its popularity and the innovation related thereto have greatly increased the accuracy and therefore the photorealistic quality of the images printed, while at the same time attempting to lower the costs of ownership of large format full color digital print engines. While the types and numbers of inks, and ink jet cartridges, usable with such printers have increased thereby increasing the complexity of controlling interaction among the inks, cartridges, and printing medium, reduced costs of ownership and ease of serviceability continue to drive a large amount of innovation in this field of endeavor. Thus, a continued need exists in the art for low cost and at the same time technically advanced, highly accurate means of performing wide format color digital ink jet printing.